Prepare Muffin Pan:

Coat a 12-cup muffin pan with fat and dust with flour. Set oven to 350°F (175°C).

Make Muffin Batter:

Using an electric mixer with paddle attachments, cream the soft butter or margarine until smooth. Slowly incorporate sugar, vanilla sugar, and salt, beating until thoroughly combined. Add eggs one at a time, beating each for roughly 30 seconds on high speed. Combine flour and baking powder, then fold into the batter on medium speed in two separate additions, mixing just until incorporated.

Bake Muffins:

Distribute batter uniformly among the muffin cups. Position the pan on the oven rack and bake for around 20 minutes.

Cool and Slice Muffins:

Allow muffins to cool in the pan on a wire rack for about 10 minutes. Then loosen them from the cups, invert onto a parchment-lined wire rack, and let cool fully. Slice each muffin horizontally into two halves.

Prepare Pudding Mixture:

In a bowl, whisk 400 ml (1 2/3 cups) milk with both packets of pudding cream for approximately 1 minute.

Make Buttercream:

In a separate bowl, beat 200 g (7 oz) soft butter with an electric mixer until creamy. Gradually add the pudding mixture a tablespoon at a time, mixing thoroughly after each addition. Ensure both butter and pudding are at room temperature to avoid curdling. Fill a piping bag fitted with a star tip (about 8 mm / 1/3 inch diameter) with the buttercream.

Assemble Muffins:

Stir the jelly until smooth or push the jam through a sieve. Spread the jelly or jam on the bottom halves of the muffins. Pipe roughly half of the buttercream onto the bottoms, then place the top halves back on.

Decorate and Chill:

Pipe the remaining buttercream in a swirl on each muffin. Evenly scatter hazelnut brittle over the buttercream. Rinse and sort the raspberries, then set one on each muffin. Refrigerate until ready to serve.